Romans 4:
C1 = If there is no law, then there is no transgression.
⇒ if there is transgression, then there is law = C2
Romans 5:
By C2, it follows that there was law before Moses.
What kind of law?
Romans 2:
In what sense was sin not imputed to those who lived before the Mosaic Law?
Paul's point was that, in one law or another, sin has always counted since the time of Adam.
This is a self-evident statement:15b where there is no law there is no transgression.
C1 = If there is no law, then there is no transgression.
⇒ if there is transgression, then there is law = C2
Romans 5:
Before Moses' law, there was the flood and death. There was sin and transgression.13 To be sure, sin was in the world before the law was given, but sin is not charged against anyone’s account where there is no law.
By C2, it follows that there was law before Moses.
What kind of law?
Romans 2:
It was the law written in people's consciences, their moral sense of right and wrong.14 Indeed, when Gentiles, who do not have the [Moses'] law, do by nature things required by the law, they are a law for themselves, even though they do not have the law. 15 They show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts sometimes accusing them and at other times even defending them.
In what sense was sin not imputed to those who lived before the Mosaic Law?
Paul's point was that, in one law or another, sin has always counted since the time of Adam.